1. Field of the Invention
The present invention relates generally to a circuit for correcting a deflection distortion, and more particularly, to a circuit for generating a parabolic wave used for correcting a deflection distortion in an image display apparatus having a cathode ray tube.
2. Description of the Background Art
FIG. 1 is a block diagram showing the conventional deflection distortion correcting circuit used in an image display apparatus having a cathode ray tube. FIG. 2 is a circuit diagram of an integration circuit which is coupled to a PWM circuit 11 shown in FIG. 1. FIGS. 3A, 3B, 3C, 3D, 4A, 48 and 4C are waveform charts of outputs from main circuits constituting the deflection distortion correcting circuit:
Basic clock pulses generated by a basic clock generator are inputted into a frequency divider 2 through a basic clock input terminal 1. The frequency divider 2 divides the frequency of the basic clock pulse by N (N: an integer). The output of the frequency divider 2 is a pulse signal as shown in FIG. 3A. The basic clock pulse having the divided frequency is inputted into a counter 3 and a flip-flop circuit 8. A vertical synchronizing signal is separated from an image signal which is received by the image display apparatus. The separated vertical synchronizing signal is inputted into the counter 3 through an input terminal 4. The counter 3 counts up the output pulses of the frequency divider 2 over one vertical scanning period. The counter 3 is reset at every rise of the vertical synchronizing signal. The counter 3 outputs an upward step-like signal as shown in FIG. 3B. The count value is outputted to a subtracter 6.
A center value which is previously set for determining the period of the parabolic wave is inputted into the subtracter 6 through an input terminal 5. The center value is specifically a value which represents the number of output pulses of the frequency divider 2 counted by the counter 3 during a period corresponding to a half of one vertical scanning period. The subtracter 6 subtracts the count value which is inputted from the counter 3, from the center value, and outputs the subtraction result to an adder 7. The subtracter 6 outputs a downward step-like signal as shown in FIG. 3C.
The adder 7 adds the output data from the subtracter 6 to the output data from the flip-flop circuit 8, and then outputs the addition result to the flip-flop circuit 8. The flip-flop circuit 8 determines the output of the adder 7 for each period of the output pulse of the frequency divider 2, and outputs the determined output to the adder 7 and an adder 10. The output from the flip-flop circuit 8 has a step-like waveform as shown in FIG. 3D, and has an arcuate waveform as a whole in each vertical scanning period.
The operation in which the output of the subtracter 6 is added to the output of the flip-flop circuit 8 in the adder 7 and the addition result is again determined by the flip-flop circuit 8 is repeatedly performed at a period of the output pulse of the frequency divider 2, so that the output of the subtracter 6 is gradually integrated.
The adder 10 receives control data for controlling the direct-current components of the parabolic wave, via an input terminal 9. The control data is added to the output data from the flip-flop circuit 8 in the adder 10. The addition result is outputted to a pulse width modulation (hereinafter abbreviated as PWM) circuit 11.
The PWM circuit 11 generates a PWM signal shown in FIG. 4B in which the pulse width is varied in accordance with the inputted added value. The PWM signal is outputted to an integration circuit 120 shown in FIG. 2. The integration circuit 120 is constructed by combining a resistance 120a with a capacitor 120b. The integration circuit 120 integrates the PWM signal or converts the PWM signal into a smooth parabolic wave shown in FIG. 4C, and outputs the parabolic wave.
The thus obtained parabolic wave is processed so as to change the magnitude of a horizontal deflection current in the image display apparatus having a cathode ray tube. Thus, deflection distortion is corrected.
In the above-described prior art deflection distortion correcting circuit, however, when the PWM signal is converted into the parabolic wave in the integration circuit 120, the waveform is distorted, for example, as shown by a broken line in FIG. 4C, by the influence of the time delay of the integration circuit. This produces a problem in that it is difficult to obtain a predetermined parabolic wave indicated by a solid line in FIG. 4C.